Non-volatile memory devices, such as flash memory devices, are memory devices that can store information even when not powered. A flash memory device stores information in a charge storage layer that is separated from a “control gate.” A voltage is applied to the control gate to program and erase the memory device by causing electrons to be stored in, and discharged from the charge storage layer.
A control dielectric is used to isolate the control gate from the charge storage layer. It is desirable for the control dielectric to block charge flow between the charge storage layer and control gate. High-k dielectric layers can serve as efficient charge-blocking layers. They have been used as the control dielectric layer for flash memory devices, such as Samsung's TANOS devices, to enable the down-scaling of flash memory devices below 40 nm. The control dielectric layer may be a single layer of Al2O3, typically with a thickness of less than 20 nm. However, Al2O3 does not completely block charge transport and leads to program and erase saturation at lower voltage windows.
What is needed are improved, longer lasting non-volatile memory devices, with improved charge blocking characteristics. Furthermore, multi-state memory devices exist, which can store more than one bit of information per memory cell. What is needed are improved multi-state memory devices that can store multiple bits per cell with relatively large program/erase voltage windows of operation.